1. Field of the Invention
The present invention relates to a rotational frequency control apparatus of an internal combustion engine wherein the intake adjustment and rotational frequency adjustment operations are realized in higher speed through utilization of a loop for adjusting the intake amount to a target value and a loop for adjusting a rotational frequency to a target value.
2. Description of the Prior Art
Unloaded rotational frequency of an internal combustion engine has been controlled to a predetermined constant rotational frequency. Objects of such rotational frequency are to set unloaded rotational frequency to a lower value in order to suppress, as much as possible, fuel consumption under the unloaded condition and to suppress variation of rotational frequency due to disturbances. Therefore, the rotational frequency control is always required to have quick and highly accurate controllability. Factors causing varying rotational frequency are roughly divided into a primary factor due to variation of unloaded loss of the engine itself and variation of thermal efficiency of engine and a secondary factor due to variation of adjustment gain internally existing in an intake amount adjusting means used for adjusting variation of rotational frequency by the primary factor and variation of density of atmospheric air considered as the intake source.
Therefore, as proposed in Japanese Laid-open Patent No. 162340 1984, there is prior art in which a rotational frequency is controlled to the target value by controlling an intake amount adjusting means in accordance with an adjusting signal based on deviation between the target value of rotational frequency and actual value and an adjusting signal based on deviation between the target intake amount or intake pipe pressure and actual value.
According to the prior art described above the following advantage can be attained. Namely, since the adjusting signal (rotational frequency adjusting signal) based on deviation between the target value of rotational frequency and actual value responds to the primary factor of rotational variation while the adjusting signal (intake adjusting signal) based on deviation between the target value of intake amount or intake pipe pressure and an actual value responds to the secondary factor, a rotational variation can be adjusted more quickly and accurately than in the case of feedback control based only with the rotational frequency.
As explained heretofore, the prior art described above proposes employing an intake amount adjusting loop in order to self-correct an error of a rotational frequency control means itself.
However, the rotational frequency control apparatus of the internal combustion engine of the prior art mentioned above also provides following disadvantages.
First, under the unloaded condition, a rotational frequency is controlled to the target value but when the engine is set to the loaded condition after the throttle valve opens a little, if an adjustment is enabled, the primary factor and secondary factor are no longer adjusted, an intake amount of the engine changes suddenly, increase or decrease of the rotational frequency becomes abnormal, remarkably deteriorating operationability.
Particularly for the secondary factor, the aging due to initial fluctuation of control means or clogging becomes large and it becomes a problem not to be neglected.
Moreover, as explained previously, the rotational frequency control means is provided with an intake amount adjusting loop in order to correct an error thereof by itself in addition to a rotation frequency adjusting loop in the conventional apparatus mentioned above. In case the engine rotates, for example, at the rotational frequency determined by a target rotational frequency generating circuit corresponding to the engine temperature, such conventional apparatus has also been accompanied by such a defect that eventually its response characteristic cannot exceed the response characteristic of the apparatus provided with only the rotational frequency adjusting loop, since the difference between the target rotational frequency and actual rotational frequency of the engine is corrected by a rotational frequency operation unit.